Or you could move up to a proper monitoring system where the mixer feeds one or more powered monitor speakers for the benefit of the performers. You could mic the piano's speakers, in the same way as a guitar amp is often miked to be put through the PA. I don't know what gave you the idea that audio is available at the USB port. It's easy enough to bypass this switching internally - at least it is if you can get access inside the case, and there's room to use a soldering iron! You'll lose your warranty of course. Inserting the headphone plug operates switches within the socket that isolate the internal speaker. But I don't feel that's going to be a good idea since the warranty would be lost. Obviously, one option would be to hack into the piano and add yet another jack output coming directly from the internal speakers' cables. This is the relevant page from the manual: I believe this is a common issue, but I can't seem to find any solutions.

Taking this specific case (Yamaha YDP 143), is there any hope that we can use the USB output and some other device to stream the audio data from the USB cable (which should be shorter than 3 meters) through another long cable going to the main interface? Is there a way to use both the headphones output and internal speakers in a stage setup when using a digital piano which doesn't have a specific 'AUX OUT' output? A cable is used to connect the piano to the main interface, currently sending the audio data via the headphones output.

We want the piano speakers to be used always, even when a jack enters in the headphones output. It's a digital piano which has USB output (for MIDI and audio data) and two outputs for headphones.

However, things get a little bit tricky when there is a stage setup. This is a common problem I had with many keyboards, and I think it's a standard, somehow: when connecting the headphones to a piano, the internal speakers are disabled.

Speakers and headphones simultaneously